An excellent move. More than going for b5-b4, which can be met by c3-c4, Firouzja secures the c4-square for the bishop.
8. b5c3 By going up and down with the knight, the b6-square has been weakened and I think this was Keymer's plan. He now can consider capturing on b6 in a better version, but Firouzja also has some ideas. 8... b4c3  9. d2c3  9... b6a4  10. g1e3 